Mahale Mountains National Park occupies the mountainous bulge that protrudes into Lake Tanganyika along its eastern shoreline. The mainly grass-covered mountain ridge that runs parallel to the lake is forested in parts with rich gallery forest extending down the many watercourses. Some of these forested valleys have extensive stands of bamboo. The lower slopes are cloaked in tall Brachystegia woodland which dominates the western and drier eastern areas. The south-western and southern tip of the peninsular is dominated by smaller Brachystegia woodland.

The area is relatively poorly known ornithologically. Bird species list includes only 214 species and these include three Guinea - Congo Forests biome species. An endemic subspecies of the globally threatened Apalisargentea is present here. Endemic subspecies of Phyllastrephusflavostriatus, Andropadustephrolaemus, Platysteiraconcreta, Alethepoliocephala, Sheppardiabocagei and Phylloscopusruficapilla, and an endemic race of Nectariniaregia also occur. A new subspecies of Anthoscopuscaroli was described from the site in the 1960s. The highland-forest Poeopterastuhlmanni is only known in Tanzania from Mahali, as is the enigmatic Bradypterusalfredi

Eurasian migrant birds, raptors, acacia associated birds, and three species of bird, which are endemic to Tanzania: the ashy Stirling, rufous-tailed weaver and black-collared lovebird.